One of the most surprisingly influential books I ever read was “The Memory Book” by Harry Lorayne and Jerry Lucas (the only book written by a professional magician and a professional basketball player that I am aware of!). “How to Develop a Super-Power Memory” by Harry Lorayne is probably better known. There are also regular memory competitions and many “memory athletes” have publically discussed their techniques.
Memory techniques were standard curriculum centuries ago (before various recording media were invented) but it’s pretty much unheard of now, so people don’t realize just what the human brain is capable of.
Champion memory athletes regularly memorize hundreds of random digits, multiple shuffled decks of cards, hundreds of historical facts, an entire auditorium of peoples names, entire phone books (remember them?), or even entire city skylines in just a few minutes. It’s truly shocking the first time you see it done.
Mnemonics are intended to build associations in your brain, in particular for short-term memory.
More specific to Kanji and WK, you’ll depend on mnemonics less and less as you move items through the stages, likely not depending on them at all when you “burn” an item: by that time, you’ve built a long-term association in your memory and don’t need to rely on “tricks.”
Both memory books and pretty much all the memory athletes agree on what makes a good mnemonic association: something VISUAL that is as exaggerated, absurd, grotesque, embarrassing, or sexual as possible. The best associations are deeply personal, often impossible to explain to others, and almost always too embarrassing to explain even if you could!
Here’s a pretty good treatise on the subject of memory associations.
The WK-provided mnemonics can never be as good as the ones you create yourself, but, despite all the grumbling I read on this forum about the “silly” default mnemonics that WK created, I think they’ve done an amazingly good job.
It takes time and effort to create decent mnemonics, and the defaults that WK provides are usually good enough to jump-start the process. I appreciate not having to even expend the effort to create a mnemonic for every single item here.
I’ll usually start with the WK-provided mnemonic (because I’m lazy) but if I find I keep missing reviews for an item, then I’ll take the time to create my own.
As the article I linked to above points out, the absolute worst thing you can do is to create mnemonics that are logical, reasonable, and boring!